Lessons on Soil | Page 7

jars and still keep on till the liquid is no longer muddy but nearly
clear. The part of the soil that remains behind and will not float over
into the jars is at once seen to be made up of small stones, grit, and
sand. Set the jars aside and look at them after a day or so. The liquid
remains muddy for some time, but then it clears and a thick black
sediment gathers at the bottom. If now you very carefully pour the
liquid off you can collect the sediments: they are soft and sticky, and
can be moulded into patterns like clay. In order to see if they really
contain clay we must do the experiment again, but use pure clay from a
brick yard, or modelling clay, instead of soil. The muddy liquid is
obtained as before, it takes a long time to settle, but in the end it gives a
sediment so much like that from the soil, except in colour, that we shall
be safe in saying that the sediments in the jars contain the clay from the
soil. And thus we have been able to separate the sticky part of the
soil--the clay--from the gritty or sandy part which is not at all sticky.
We may even be able to find out something more. If we leave the soil
sediment and the clay sediment on separate tin lids to dry, and then
examine them carefully we may find that the {7} soil sediment is really
a little more gritty than the clay. Although it contains the clay it also
contains something else.
When the experiment is made very carefully in a proper way this
material can be separated from the pure clay. It is called silt, but really
there are a number of silts, some almost like clay and some almost like

sand; they shade one into the other.
If there is enough grit it should be weighed: we obtained 14 decigrams
of grit from 10 grams of our top soil and 17 decigrams from 10 grams
of bottom soil. We cannot separate the clay from the silt, but when this
is done in careful experiments it is found that the subsoil contains more
clay than the top soil. We should of course expect this because we have
found that the subsoil is more sticky than the top soil. These results are
put into the columns as before so that we can now see at once how
much of our soil is water, how much can burn away, how much is grit,
and how much is clay and other things.
What would have happened if the sample had been dug out during
wetter or drier weather? The quantity of water would have been
different, but in other respects the soil would have remained the same.
It is therefore best to avoid the changes in the amount of water by
working always with 10 grams of dried soil. The results we obtained
were:--
Top soil Subsoil Weight of dry soil before burning ... 100 100
decigrams " " " after " ... 92 97 " --- --- The part that burned away
weighed ... 8 3 Weight of grit from 10 grams of dried soil 17 19 "
The columns are given in Fig. 3.
{8}
[Illustration: Fig. 3. Columns showing what 100 parts of dried soil and
subsoil were made of]
Summary. The experiments made so far have taught us these facts:--
1. Soil contains water, grit or sand, silt, clay, a part that burns away,
and some white chalky specks.
2. The top layer of soil to a depth of about eight inches is different from
the soil lying below, which is called the subsoil. It is less sticky, easier
to dig, and darker in colour. It contains more of the material that burns

away, but less clay than the subsoil.
3. When soil is dried it is not sticky but hard or crumbly; as soon as it is
moistened it changes back to what it was before. But when soil is burnt
it completely alters and can no longer be changed back again.

[1] See p. xiv for explanation of the figures in square brackets.

{9}
CHAPTER II
MORE ABOUT THE CLAY
Apparatus required.
Clay, about 6 lbs.; a little dried, powdered clay; sand, about 6 lbs. Six
glass jars or cylinders [2]. Six beakers [1]. Six egg-cups [1]. Six
funnels and stands [2]. Six perforated glass or tin disks [2]. Six glass
tubes [2]. Two tubulated bottles fitted with corks. Some seeds. Six small
jars about 2 in. x 1 in. [2]. Bricks. The apparatus in Fig. 9. Pestle and
mortar.
We have seen in the last chapter that clay will float in water and only
slowly settles down. Is